The present invention relates to a reset circuit for detecting a voltage drop of a power supply for a logic circuit such as a microcomputer and to prevent unstable operation of the logic circuit or a system in which the logic circuit is applied.
Generally, in a system in which a logic circuit such as a microcomputer is applied, an erroneous operation could occur when the power supply voltage drops. Such an erroneous operation is especially problematical where the logic circuit is applied in an area such as an automobile controller, and to a machine tool controller where high reliability is required. To prevent erroneous operation, a reset circuit is provided to detect the voltage drop and to reset the logic circuit to a safe condition.
A conventional reset circuit of this kind comprises a circuit for detecting a voltage drop in the power supply and resets the logic circuit responsive to the voltage detecting circuit. The voltage detecting circuit can be formed of a special IC (integrated circuit), or of a combination of a reference diode, an operational amplifier or other discrete circuit elements.
However, the above reset circuit is complicated in configuration, expensive and occupies a large space. Moreover, the detection voltage is fixed. For instance if the circuit is fabricated for operation with a 5 V power supply, it cannot be used for operation with a power supply having another voltage value such as 4 V.
Moreover, when the power supply voltage repeatedly drops and is restored a number of times in a short time, due for instance to chattering that occurs when the power supply is connected or the power supply switch is closed, the logic circuit under control of the reset circuit will repeatedly reset and operate. As a result, the operation of the system in which the logic circuit is applied is unstable. To prevent repetitive resetting, a capacitor with a large capacitance such as several microfarads, can be connected in the power supply circuit. But such a large-capacitance capacitor cannot be built in an IC and increases the space required and the cost.